Various vehicles or machines such as wheel loaders, cars, trucks and other wheeled machines, use powertrains that include a torque converter between the engine and transmission. The torque converter can be used in a locked or an unlocked mode when the machine is moving, which provides for a desirable torque control feature for some machines that may encounter relatively immovable obstacles. For example, a wheel loader may push a bucket against a pile of material, a ripper may encounter a boulder, and so forth. Torque converter slippage is relied upon to provide a transmission output torque control benefit, for example, when contact with the obstacle is initiated, because it results in limiting transmission input torque and reducing the possibility of low engine lug issues, such as engine underspeed or stalling. However, torque converter slippage and other losses have been found to increase overall fuel consumption of the machine when compared to other power transmission methods such as continuously variable transmissions (CVTs).
CVTs provide a continuously variable torque capability, which is an improvement over a traditional torque converter/transmission powertrains. A typical CVT employs a torque controlling element that provides a continuously variable torque or speed transmission capability. One known application of CVTs for machine use is embodied as a split torque transmission, which includes a drive train that is powered by dual inputs—one input being a torque- or speed-controlled input, such as from a hydraulic variator, and the other being a direct power input from an engine. These two inputs are combined in one or more planetary gear arrangements, each of which includes outputs driving the various gear ratios of the transmission.
Split torque transmissions, however, do not realize the inherent torque control benefits of a torque converter based powertrain. Accordingly, it is desirable to accurately control the variator, especially when rapid changes in the torque passing through the transmission occur, for example, when an obstacle is encountered. Control of the variator under such conditions requires the swift detection of torque changes, which in most current implementations is left to the perception of the operator. Thus, in most current implementations, the operator must cut or reduce machine torque when the operator perceives that an immovable object has been encountered. As can be appreciated, there is a lag time between the instant when the machine begins to encounter the obstacle until the time when the operator has sufficiently reduced the torque command to the transmission and the transmission has adjusted its operation accordingly. This time lag may cause the machine to rock, thus causing operator discomfort, system inefficiency, and/or increased drive train wear. In some instances, such time lag may cause engine underspeed and/or stall.